1. Field of Invention
The invention generally relates to a method of enhancing edge quality when processing an image, and in particular relates to a method of edge enhancement for halftone image.
2. Related Art
For printers or scanners, there are specific sizes of printing or scanning. In a certain size area, more pixels make a higher image quality. The pixel resolution is measured by dots per inch (dpi). In monochromatic printing, pixels are non-black or white. In the dpi solution, a pixel may include several dots. Usually, when a pixel is to be printed, all the dots of the pixel are printed. Likewise, when a pixel is not to be printed, all the dots of the pixel are not printed.
However, if the resolution of the image output apparatus is relatively low, the edge of the image object is found to be step-shaped of one grid by another grid through the naked eye, and a smooth edge as that of the original image object cannot be obtained. Therefore, various solutions are proposed to reduce the jagged effect on the edge of the image object, such that the image output by the image output apparatus can match with the real subject.
As for Edge Enhancement Method and Apparatus for Dot Matrix Devices disclosed in U.S. Pat. No. 5,029,108, the dotting state of the pixels of an image edge and that of the surrounding pixels are compared with a plurality of predetermined pictures, to determine which predetermined picture the pixels of the image edge match with, thus it is determined how to dot the pixels at the image edge to smooth the output image edge and to allow the image more accurately match with the original one. The enhancement of central pixels of each predetermined picture will be modified differently according to the dotting state of the surrounded pixels, for example, dotting at ¼ to the left of the central pixel, dotting at ⅔ to the right of the central pixels, etc
However, the method mentioned above is only suitable for single color image output (namely, the black and white image output). When executing the color image output of the image output device, since each of the color images are composed of the planes of four different colors (cyan, magenta, yellow, and black; CMYK), Thus, in executing the supplementary dotting of the pixels on the edge of the image, the planes of all the four colors must be taken into consideration. Presently it doesn't have any mechanism, which can be utilized to determine the planes, of which colors must be utilized to implement the supplementary dotting of the pixels on the edge of the image. However, if the planes of all the four colors are used to perform the supplementary dotting, then the color displayed on this pixel will be black, thus it may be different from that of the original subject. Nevertheless, there is just no way to know which color may be used to implement the supplementary dotting.
Further, many printing and display devices can only produce binary outputs, therefore, color images have to be transformed into binary outputs. The transforming is called “halftone”. In halftone process, the error diffusion is a usual method to get higher quality binary outputs. Because halftone output is decreased of its quality during the color level degression, there are many different techniques trying to improve the problem.
In U.S. Pat. No. 6,424,747, a smoothing circuit smoothes lookup tables generated by a color conversion table generator, using a filter function selected from a filter function storage circuit, and supplies smoothed lookup tables to a color processing circuit. However, the method of direct changing the color value may influence the image color reality. In U.S. Pat. No. 6,201,613, an image enhancement of halftone images is performed by subjecting the halftone image to a low-pass filter so as to smooth out halftone variations between adjacent pixels. However, the smoothing is limited due to processing of already-processed halftone image only. U.S. Pat. No. 6,061,145 also discloses a halftone image processing method in which multi-gradation image data is quantized to be converted into quantized pixel patterns, a smoothing portion of a quantized image is detected and a contour of the quantized image is corrected by interpolating pixels of the detected smoothing portion. The quantized pixel pattern of the smoothing portion in the detected quantized image is replaced by a pixel pattern for smoothing stored in a memory. Since the detecting and replacing is a two-step process, it takes more time. Further, more preset patterns cost more detecting time and larger memory size, so it is not practicable. U.S. Pat. No. 5,757,976 provides a set of error filters having different sizes and associated weighted coefficients for diffusing quantization errors among neighboring pixels in predetermined tonal areas of an image to achieve a smooth halftone image quality. However, the error filters can only influence the noise and repeated patterns of halftone process. They don't have effective improvements to original images of lower quality.
Therefore, the conventional methods mostly aim at the overall image quality improvement, or just the monochromatic image edge improvement. As for color image output devices, how to take enhancements to the edge of an image, and to make the edge of output image smoother and conformed to the original color, actually remains an unresolved difficult problem.